The "kid's" way of understanding the expanding universe is that: "space" is totally "ordinary", and all the galaxies are expanding through it (like an explosion). Of course, that's wrong.

The usual better explanation is that "space itself is expanding." (Of course, on scales below clusters, gravity pulls "smaller" structures together.)

An even more up-to-date explanation is that the conceptual "metric of space" is "expanding" (here's a typical pedagogic example) which can perhaps be summarized as the "scale is changing".

So ... distant objects are redshifted.

But why? Everything's just expanding -- the very metric of spacetime is expanding.

Indeed, it would seem to me that you would only see redshift (or if you prefer, time dilation of far-away things) strictly in the case of "everyday" motion within the metric of space; the very idea of the actual "metric of space changing!" would seem to be that, those of us internal to that metric of space would have no clue that any such expansion is happening: the scale is just changing for everything.

What's the best way to understand this?

Imagine simply a meter cube in a video game with a few things in it. There is no exterior, it is the universe. I expand the entire thing...

enter image description here

{note...of course, obviously, the 'outside' (shadows etc.) added by the 3D presentation software to clarify the PNG here, have utterly no meaning and do not exist in any way}

enter image description here

... to all the beings inside, I believe absolutely nothing has changed, there'd be no redshift between the objects there.

What's the deal?

Note too this somewhat similar (related?) question, which came up with the recent 2016 gravitational wave discovery:

How is it that distortions in space can be measured as distances?

  • $\begingroup$ I believe you have both. A certain amount of expanding space, and objects moving relative to that space. Redshift restored. $\endgroup$ – Floris Dec 12 '15 at 2:06
  • $\begingroup$ @JoeBlow : "the scale is just changing for everything" is not true : the fine structure constant seems to be a real constant ( Planck satellite & Wmap ) , then , at least , there is no evidence that atoms expand. $\endgroup$ – user46925 Dec 12 '15 at 2:45
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    $\begingroup$ Hi @Floris ... hmm, I'm not sure if I follow you. Of course, there is ordinary everyday doppler or gravitational redshift, and, there is cosmological redshift (caused by "the metric expansion of the universe"). (Hence, many questions on how to tell the two apart ... example ) My question is of course only about cosmological redshift ... my question, why should THE WHOLE DAMNED METRIC expanding, be detectable, at all, within that system? $\endgroup$ – Fattie Dec 12 '15 at 3:51
  • $\begingroup$ @JoeBlow : Anna V answered this question. Expansion theory needs less changes. There is a significant correlation between the redshift as a distance and characteristics of observed objects that reflect their ages $\endgroup$ – user46925 Dec 12 '15 at 12:38
  • $\begingroup$ Hi igael - she DID answer my question, exactly by stating that "atoms do not take part in the metric expansion." Waves get metric-expanded: if atoms did take part in the metric-expansion, we would see nothing, no change. I think. Indeed, just as you say "atoms do not expand". $\endgroup$ – Fattie Dec 12 '15 at 16:01

What are the observational/experimental facts:

1)Atoms have definite spectra, with a fixed pattern, a fingerprint of the atom

2) The further away ( measured by luminocity) galaxies all around ours the more shifted the fingerpring pattern towards the red part of the spectrum.

3) This happens uniformly all around.

The model that fits these facts is General Relativity, which predicted the behavior

In the hierarchy of forces , the gravitational force is the weakest. This assures that atoms, matter in general up to the size of galaxies keep their structure, the raisin bread analogy. Gravity is strong enough to keep even clusters of galaxies unaffected and given some assumptions on the energy density and solution of the general relativity equations gravity can fight the expansion and lead to the big crunch,.

Photons are elementary particles that have to obey locally energy and momentum conservation. The expansion of the universe changes their momentum and thus the atomic spectra arrive shifted towards the infrared.

the very idea of the actual "metric of space changing!" would seem to be that, those of us internal to that metric of space would have no clue that any such expansion is happening: the scale is just changing for everything.

It is the fact that matter is bound by forces that are not affected by the expansion that allows us to measure the expansion. Otherwise you are correct, our atoms would also be expanding and we would see no shift in the atomic thumbprints.

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    $\begingroup$ @BenitoCiaro All our explanations are ad hoc, they are called mathematical models; and they are accepted not because of their descriptive power ( that would just be mapping) but because of their predictive power . Theoretical models stand as long as new data validates them. $\endgroup$ – anna v Dec 12 '15 at 7:21
  • $\begingroup$ Hi Anna V First thanks as always. Let me try to get to the heart of the matter. As you say (1) matter is bound by forces that are not affected by the expansion (2) that allows us to measure the expansion (3) Otherwise our atoms would also be expanding and we would see no shift. The confusion for me is, the only thing that happens to "expand" is structures bigger than clusters. With other types of motion (caused by gravity, explosions, etc etc) we extremely simply say "it happens to be 'moving'" and that, 'motion', is what causes the red shift in that case. However....... $\endgroup$ – Fattie Dec 12 '15 at 15:11
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    $\begingroup$ ....... it appears that strictly in the case of structures-bigger-than-clusters, instead of just saying "they are moving apart" (for some reason - an as yet unknown force ... whatever ... ), we assert instead that the very nature of the epistemological underlying coordinate system is scale-changing and that the entirety of "spacetime" is indeed undergoing metric expansion. So, that's what I'm wondering here. $\endgroup$ – Fattie Dec 12 '15 at 15:16
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    $\begingroup$ Well in the raisin-dough example, the raisins stay a fixed size. (Right??) If the raisins also increased in size (precisely as in my visual example: notice the two raisins) then there is no redshift. Indeed, you have explained to me that atoms are NOT expanding which I suppose is precisely the exact answer to my specific question here. $\endgroup$ – Fattie Dec 12 '15 at 15:37
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    $\begingroup$ Note too that: it is always pointed out that due to GRAVITY, although the spacetime metric is expanding, structures below clusters are NOT expanding. However. I now realize this has absolutely nothing, whatsoever, to do with the question here ("how come redshift if everything's expanding?") The reason for redshift is because strictly ATOMS are not expanding. Say gravity was weaker and indeed clusters and galaxies, even planetary systems WERE expanding. That would make no difference. We'd stlll see redshift, because ATOMS are NOT expanding with the spacetime metric. I think. $\endgroup$ – Fattie Dec 12 '15 at 15:40

It's pretty easy to explain if we take a classical view of an electromagnetic wave. As an EM wave from a distant star propagates towards us, the space it propagates through is expanding. Since the space is expanding, the peaks & troughs of the EM wave are getting farther apart from each other. That corresponds to an increase in wavelength and a decrease in frequency; a redshift arises.


I think you're imagining "the very metric of space is expanding" to mean that the definition of distance is changing$^{†}$, but that's not the case. The "metric of space expanding" means the distances themselves are changing. As an electron and a proton are subjected to an expansion of the space them, the electrostatic attraction between them (speaking in a roughly non-quantum framework) pulls them "back together" and maintains the size of the atom.

The "rubber sheet" analogy is a little perilous, but in this case I think it's apt. Two objects on an expanding rubber sheet will experience an increase in the distance between them, but not if they're connected by a spring - in that case, they will maintain their relative distance.

It's not a magic exemption that allows the atoms of our measuring apparatus to be unaffected by an expanding universe; they're unaffected because the electrostatic interactions and associated quantum mechanics that determine the behavior of subatomic particles is not changed by the expansion of space.

$^{†}$that would just mean the universe is changing from one set of units to another, which would (as you point out) be physically and philosophically undetectable, and therefore meaningless.

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    $\begingroup$ But the thing is, if (that's IF) the measuring equipment and literally everything was expanding - as one would perhaps expect from an astounding pedagogical exposition like "the very metric of spacetime is expanding" - there's no change whatsoever. $\endgroup$ – Fattie Dec 12 '15 at 15:30
  • $\begingroup$ I see your point, but I would argue that measuring equipment is made of particles held together by electrostatic and nuclear forces. The same can't be said for the peaks of an electromagnetic wave - there is no restoring force maintaining the scale of an EM wave. Does that resolve your objection? $\endgroup$ – Brionius Dec 12 '15 at 15:38
  • $\begingroup$ Hmm, if literally THE METRIC OF SPACETIME (for God's sake!) was actually expanding -- which indeed is nothing more than saying the "scale is changing", then very much the "measure" of distance between the peaks (or - any measure!) would just be "the same" - there's no redshift. But indeed, this would appear to be all resolved: it seems that when we say "the very metric of spacetime is increasing" we actually mean "BUT NOT ATOMS". So, we can sort of "magically" measure the otherwise meaningless change in "scale - distance" between those peaks: because atoms are conveniently not changing :O $\endgroup$ – Fattie Dec 12 '15 at 15:47
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    $\begingroup$ I updated my answer to hopefully address your frustration about atoms. $\endgroup$ – Brionius Dec 12 '15 at 16:18
  • $\begingroup$ BTW if you're still there, this was a great answer, @Brionius ! $\endgroup$ – Fattie Dec 9 '19 at 19:39

FYI Interestingly I found this article,


where the authors address

the exact issue of this question,

and indeed whether the whole pedagogical idea of "space expanding," is crap.

For example, section 2.6.2, is a question identical to the OP here.

2.6.2 Is everything expanding?

An extension of the argument against global expansion given in section 2.2 is that is should be undetectable, since everything will simply expand with it.

They essentially go on to say that because atoms don't expand we can measure redshift, eg,

enter image description here

Which does seem to be the asymptotic answer here.

  • $\begingroup$ "it should be indetectable": well, depends if constants keeps constant or not ( c, thin structure, etc, especially all that have lengths involved somewhere in their definition). $\endgroup$ – Fabrice NEYRET Dec 12 '15 at 22:22
  • $\begingroup$ If certain constants (or anything) change over time you would be able to see that change by looking at the past, as you suggest. In any event, the question at hand, the OP, is why is the metric-expansion detectable, since, detectors/etc themselves would expand. Answer: atoms don't expand under the metric-expansion. $\endgroup$ – Fattie Dec 12 '15 at 23:08
  • $\begingroup$ My point about constants was just the opposite situation: if the metric scaled but not the constants, you would have the impression that the constant change (e.g. c decrease), which can be detected as you point out. $\endgroup$ – Fabrice NEYRET Dec 13 '15 at 2:20

The "kid's" way of understanding the expanding universe is that: "space" is totally "ordinary", and all the galaxies are expanding through it (like an explosion). Of course, that's wrong.

It's not wrong. There is no difference in general relativity between "expansion of space" and simple relative motion. They are the same phenomenon described with respect to different coordinates.

Here's an analogy. On a planet-sized ball of dirt (the earth without oceans, mountains, etc.), carve a bunch of straight (great-circle) foot paths from one pole to the other, all of a fixed width (say 1 meter). As you go away from the poles, the foot paths get farther away from each other, reaching a maximum distance at the equator, then reconverge to the other pole.

Now consider this situation from the perspective of polar coordinates (latitude and longitude). Each path is at a constant longitude. As the latitude changes, the paths don't move apart or together; they remain at fixed longitudinal separations, while the scale factor relating the longitude to physical distances changes.

Is the separation of paths now unobservable, because the metric itself is being rescaled? No. You don't change physical reality by choosing new coordinates for it. In polar coordinates, the coordinate width of the path decreases as the scale factor increases, meaning that the physical distance between the paths, measured using the physical path width as a meterstick, increases just as before. The decrease in coordinate width is not the result of a physical force acting against the expansion force. There is no expansion force. There are just paths of constant width that don't know or care about the properties of the coordinate system that you chose.

This is a very close analogy. FLRW cosmologies have an approximate symmetry similar to the symmetry of the earth around its axis of rotation, and FLRW coordinates are similar to polar coordinates. The cosmological time is the latitude, and the spatial position is the longitude.

The laws of physics are local. If you look at any small portion of the earth (away from the equator), the footpaths on it are diverging. That local divergence (relative motion) is all that the laws of physics actually "see". We humans recognize the overall shape, and the great circles, and choose global coordinates that respect the global symmetry. That's our choice. The universe doesn't care.

  • $\begingroup$ I would have to think about this for a long time. Not internet time, a long actual time. $\endgroup$ – Fattie Aug 30 '20 at 0:33
  • $\begingroup$ Unfortunately I still feel all answers here are, in a word, hopeless. $\endgroup$ – Fattie Aug 30 '20 at 0:34

1: It is false to say "everything is expending" : space is expending, but objects linked together by forces (like gravity) are kept at same distance. So it's wrong to confuse expansion and simple change of scale.

2: When a wave travels at some speed, front 2 has a delay as compared to front 1, and needs some time to cover the distance to reach former position of front 1. But during this time expansion has enlarged the gap, so the distance between the fronts (which is the wavelength) increases : this is the red shift.

  • $\begingroup$ Thanks Fabrice. Actually, it seems to me that gravity is irrelevant. (Gravity keeps objets smaller than clusters together.) The only key thing is whether atoms are expanding. If atoms were expanding (as in my visual example), our measuring equipment and the photons would all be really expanding, and there would be no redshift. The key would seem to be what Anna says, "Otherwise you are correct, our *atoms would also be expanding and we would see no shift in the atomic thumbprints."* $\endgroup$ – Fattie Dec 12 '15 at 15:27
  • $\begingroup$ I said any forces. Atoms are bounded by various forces, so they have no reason to expand. $\endgroup$ – Fabrice NEYRET Dec 12 '15 at 22:20

Note that in your example there would actually be red shift. The two spheres are still proportionally equivalent distances apart, which means that they have moved away from each other in terms of a fixed non-shifted frame.

  • $\begingroup$ Your example claimed that everything expanded (in your diagram) remains the same. By your frame of reference (your diagram) the distance between the two spheres has increased even though they don't appear different due to scaling. A radio signal sent from one sphere to the other would take longer (that is the meaning of "expansion") in the expanded universe than it did before the expansion - even if everything appears to look the same. This is not a matter of scaling, it is a matter of the distance having changed, even if it looks the same. $\endgroup$ – M Willey Dec 14 '15 at 6:34

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